Skip to main content

`int tan^3(3x) dx` Find the indefinite integral

Before we can begin to find the integral, we must do some rearranging using some trig identities so that we have integrals that we can work with. We will need to remember the following identities as we work through this problem:


`tan^2(x)=sec^2(x)-1`


`tan(x)=(sin(x))/(cos(x))`


So, to begin, we will split up the `tan^3` like so:


`int tan^3(3x)dx=int tan(3x)*tan^2(3x)dx`


Next, we will use the first identity above to rewrite tan^2:


`int tan(3x)*tan^2(3x)dx=int tan(3x)(sec^2(3x)-1)dx`


We will then distribute and separate it into two integrals:


`int tan(3x)sec^2(3x)-tan(3x)dx=`


`= int tan(3x)sec^2(3x)dx-int tan(3x)dx`


Now we can use the second trig identity above to rewrite the second integrand:


`int tan(3x)sec^2(3x)dx-int (sin(3x))/(cos(3x))dx`


We can now use u-substitution on each integral.  We will also need the chain rule on the 3x when finding du. So the u-substitution for the first integral is:


`u=tan(3x)`


`du=sec^2(3x)*3dx`


`1/3 du=sec^2(3x)dx`


For the second integral, we will use a v for our u-substitution so as not to confuse the two as we work through them.  So the "v"-substitution for the second integral is:


`v=cos(3x)`


`dv=-sin(3x)*3dx`


`-1/3dv=sin(3x)dx`


Now, we will substitute these back into the integrals and integrate, as shown below:


`1/3 int udu-(-1/3)int (1/v)dv=(1/3)(u^2/2)+(1/3)ln|v|+C`


We can now substitute back in for u and v to find our final indefinite integral:


`int tan^3(3x)dx=1/6tan^2(3x)+1/3ln|cos(3x)|+C`

Comments

Post a Comment

Popular posts from this blog

Is there a word/phrase for "unperformant"?

As a software engineer, I need to sometimes describe a piece of code as something that lacks performance or was not written with performance in mind. Example: This kind of coding style leads to unmaintainable and unperformant code. Based on my Google searches, this isn't a real word. What is the correct way to describe this? EDIT My usage of "performance" here is in regard to speed and efficiency. For example, the better the performance of code the faster the application runs. My question and example target the negative definition, which is in reference to preventing inefficient coding practices. Answer This kind of coding style leads to unmaintainable and unperformant code. In my opinion, reads more easily as: This coding style leads to unmaintainable and poorly performing code. The key to well-written documentation and reports lies in ease of understanding. Adding poorly understood words such as performant decreases that ease. In addressing the use of such a poorly ...
Post a Comment
Read more

Is 'efficate' a word in English?

I routinely hear the word "efficate" being used. For example, "The most powerful way to efficate a change in the system is to participate." I do not find entries for this word in common English dictionaries, but I do not have an unabridged dictionary. I have checked the OED (I'm not sure if it is considered unabridged), and it has no entry for "efficate". It does have an entry for "efficiate", which is used in the same way. Wordnik has an entry for "efficate" with over 1800 hits, thus providing some evidence for the frequency of use. I personally like the word and find the meaning very clear and obvious when others use it. If it's not currently an "officially documented" word, perhaps its continued use will result in it being better documented.
Post a Comment
Read more